If the problem is the boss' Unshaken Justice attack, then have one party member other than Yukiko use a Diamond Shield (if you have it) to reduce his damage output. Otherwise, have that member attack or use healing items. At that Hp level, he should not use his Quad coverage anymore, so have the last party member go all out on the boss with his/her best attacks. Have Yukiko use an Assault signal (again, IF you have it) to boost your damage output instead of healing if her healing skill is not good enough (or you can level grind more to get a better healing skill). If Yukiko cannot heal much and there is no Assault signal in your asset, have her attack with her fire spells. Have your MC use mediarahan if Yukiko cannot afford it as often as possible (keeping your Hp high is always the most important goal at this point). Otherwise, use Rakunda to reduce his defense. Eventually, his 20% Hp should go down after 3-5 turns. Level 55-65 should be enough for this fight.